Processes of regenerating chemical nickel plating solutions



May 12, 1959 P. BUD|N|NKAS PROCESSES oF REGENERATING CHEMICAL NICKEL PLATIN@ soLUTxoNs lFiled'JaL.

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assaut Patented May 12, 1959 PROCESSES OF REGE'NERATING CHEMICAL NICKEL PLATING SOLUTIONS Pranas Budininkas, Gary, Ind., assignor to General American Transportation Corporation, Chicago, lll., a corporation of VNew York Application January 17, 1958, Serial No. 709,622

12 Claims. (Cl. 106--1) The present invention relates to processes of regenerating aqueous chemical nickel plating solutions of the nickel cation-hypophosphite anion type, and particularly to such plating solutions employed in continuous nickel plating systems of the general character of that disclosed in U.S. Patent No. 2,717,218, granted on September 6, 1955, to Paul Talmey and William J. Crehan.

A number of suitable plating solutions are available for the present purpose, as disclosed in U.S. Patent No. 2,532,283, granted on December 5, 1950, to Abner Brenner and Grace E. Riddell; in US. Patent No. 2,658,841, granted on November 10, 1953, to Gregoire Gutzeit and Abraham Krieg; and in U.S. Patent No. 2,658,842, granted on November 10, 195 3, to Gregoire Gutzeit and Ernest J. Ramirez, as well as inl the copending application of Gregoire Gutzeit, Serial No. 376,977, filed August 27, 1953, the copending application or" Gregoire Gutzeit, Paul Talmey and'Warren G. Lee, Serial No. 478,492, tiled December 29, 1954, and in the copending application of.

Gregoire Gutzeit, Paul Talmey and Warren G. Lee, Serial No. 569,815, filed March 6, 1956, now U.S. Patent No. 2,822,294.

A chemical nickel plating solution of this type essentially comprises an aqueous solution of nickel cations and hypophosphite anions, the nickel cations being derived from nickel sulfate, nickel chloride, nickel hypophosphite, etc., and the hypophosphite anions being derived from hypophosphorous acid, sodium hypophosphite, potassium hypophosphite, nickel hypophosphite, etc. Preferably, such a plating solution comprises an absolute concentration of hypophosphite anions within the range 0.15 to 1.20 moles/liter, a ratio between nickel cations and hypophosphite anions expressed in molar concentrations within the range 0.25 to 1.60, and a pH in the range 4.5 to 11. The plating solution disclosed in the Gutzeit, Talmey and Lee application Serial No. 569,815 is particularly advantageous andcomprises, in addition to the nickel cations and the hypophcsphite anions, lactic anions and propionic anions, and having a pH in the acid range 4.4 to 5.6; and specifically this plating bath comprises an absolute concentration of hypophosphite anions within the range 0.15 to 1.20 moles/liter, a ratio between nickel cations and hypophosphite anions expressed in molar concentrations within the range 0.25 to 1.60, an absolute concentration of lacticY anions within the range 0.25 to 0.60 mole/ liter and an absolute concentration or" propionie ions within the range 0.025 to 0.060. A typical plating solution of'this type has the following composition:

In the continuous plating system, as the nickel reduc-v tion-hypophosphite oxidation reaction proceeds, the nickel cations and hypophosphite` anions are depleted with the formation of hydrogen ions; and in order to preserve substantially the initial composition of the plating solution, the same is partially regenerated, either periodically or continuously, by the addition of nickel cations, hypophosphite anions and hydroxyl anions; however, as this partial regeneration proceeds, there is a build-up in the plating solution of phosphite anions, the anions of the nickel salt employed and the cations of the hypophosphite employed. Assuming that the plating solution is initially composed, and is regenerated, employing nickel sulfate, sodium hypophosphite and sodium hydroxide, it is apparent that phosphite anions, sulfate anions and sodium cations build-up in the plating solution, as a result of the use and regeneration mentioned.

The build-up of substantial phosphite anions in the plating solution is particularly undesirable, since ultimately nickel phosphite will be precipitated therein, the tolerance of the plating solution to the presence of phosphite anions therein being dependent upon the particular composition of the plating solution, the plating solution of the composition specified above having a phosphite anion tolerance in excess of about one molar.

Nevertheless, the presence of the phosphite anions in the plating solution is objectionable, whereby there is disclosed in the copending application of Paul Talmey, Gregoire Gutzeit and Donald E. Metheny, Serial No.. 576,931, filed April 9, 1956, a process for completely, regenerating such a chemical nickel plating solution in.- volving the removal therefrom of the phosphite anions (as well as the sodium cations and the sulfate anions iny the case where the bath is composed and regenerated employing nickel sulfate and sodium hypophosphite), together with the additions of nickel cations, hypophosphite anions and hydroxyl ions. Gutzeit and Metheny is very advantageous in View of the fact that a regenerated chemical plating solution is brought back substantially to the initial composition thereof with no resulting build-up therein of phosphite anions, and the other ions mentioned. In accordance with the fundamental principle of this process, the depleted chemical nickel plating solution is first contacted by a cation exchange resin, sofas to bring about the extraction therefrom of the nickel cations; then the euent is subjected to calcium hydroxide treatment, wherein the pH of the solution is increased well into the base range bringing about the precipitation of calcium phosphite. The resulting solution is then filtered to remove the precipitated calcium` phosphite therein; and the filtrate is cooled to a temperature within the general range 0 C. to 5 C., bringing about the crystallization therein of sodium sulfate. This solution is then filtered to remove the sodium sulfate therefrom; and the resulting filtrate is reconstituted by the addition thereto of nickel sulfate, sodium hypophosphite and sulfuric acid.

While this process is entirely satisfactory for the purpose of completely regenerating the depleted plating solution, it is subject to the criticism that it involves a larger number of individual steps than are desirable, as it is emphatic in carrying out the proces that the nickel cations must be removed from the depleted plating solution prior to the treatment thereof with calcium hydroxide in the substantial base range, as otherwise nickel phosphite will be coprecipitated with calcium phosphite.

Accordingly, it is a general object of the present invention to provide an improved process of completely regenerating a chemical nickel plating solution of the nickel cation-hypophosphite anion type that involves a greatly minimized number of individual steps.

Another object of the invention is to provide an improved process of removing phosphite anions from a depleted chemical nickel plating solution of the type' noted, without the prior removal of nickel cations from This process of Talmey,`

involves an improved control of the pH of the solution during the precipitation mentioned.

A still further object of the invention is to provide an improved process of regenerating completely a chemical nickel plating solution of the nickel cation-hypophosphite anion type that is simple and economical to carry out.

Further features of the invention pertain to the particular arrangement of the steps of the process, whereby the above-outlined and additional operating features thereof are attained.

, 4 i ciated tank 13. 1n the tank 13, the filtrate is treated with a composite aqueous slurry of calcium sulfate (gypsum) and calcium hydroxide, theltrate in the tank 13 being agitated, and the addition of the calcium hydroxide being controlled so that the pH thereof is elevated into the general range 5.5 to 6.0; and particularly the pH is maintained below 7.0, so as positively to prevent the precipitation therein of nickel phosphite. Under the controlled conditions mentioned, calcium phosphite is precipitated, without any substantial precipitation ot calcium hypophosphite or any nickel salt. Preferably, the total addition of calcium ion is at least less than the stoichiometric amount thereof required'to precipitate all of the phosphite anions in order to avoid the presence of calcium ions in the regenerated plating solution.

The resulting suspension is delivered to an associated iilter 14, whereby the calcium phosphite is separated therefrom, and the resulting ltrate is delivered to an associated tank 15.

The invention, both as to its organization and principle l small, or negligible, if the total addition of calcium ions of operation, together with further objects and advantages thereof, will best be understood by reference to the following specication taken in connection with the accompanying drawings, in which:

Figure l is a diagrammatic illustration of the steps involved in the complete regeneration of a chemical nickel plating solution of the nickel cation-hypophosphite anion type, in accordance with the process of the present invention; and

Fig. 2 is a diagrammatic illustration of a continuous nickel plating system in which the plating solution mentioned may be employed.

The present invention is predicated upon the discovery that in a depleted aqueous chemical nickel plating solu-j tion of the nickel cation-hypophosphite anion type, the undesirable phosphite anions may be selectively precipitated in the presence of the desirable nickel cations and hypophospbite anions, without the coprecipitation of either the nickel cations or the hypophosphite anions, by alkaline earth cations, when the pH of the solution is properly controlled and maintained within the range 5.5 to 7.0, during the precipitation of the alkaline earth phosphite. ln accordance with the arrangement, the required alkaline earth cations are supplied jointly by the corresponding soluble salt and by the corresponding hydroxide.

For example, when the plating solution is initially composed employing nickel sulfate and sodium hypophosphite, calcium may be advantageously employed, the salt4 (calciurn sulfate) and the hydroxide (calcium hydroxide) being employed jointly to supply the required calcium cations and to obtain the desired pH.

' Referring now to Fig. l, a depleted aqueous chemical nickel plating solution of the nickel cation-hypophosphite anion type, derived from a continuous plating system may essentially comprise:

Ni++ m./1 0.08

Na+ ....m./l 1.50 HzPOz- TTI /1... -HPO3" m./1 1.25 S04-r ..-m./1 0.48 Lactic ions m./l 0.30 Propionic ions m./l 0.03 pH 4.5 to 4.7

ln the tank 15, While the solution is` agitated, an aqueous solution of sodium fluoride is addedl for the purpose of precipitating calcium uoride. This step is altogether optional, as the amount of excess calcium ions in the liltrate delivered to the tank 15 is very in the tank 13 has been properly controlled, as described. Moreover, this small quantity of calcium ions is not objectionable in the plating solution. However, the calcium ions may be removed, if desired, by the fluoride precipitation, if an excess thereof has been added. As illustrated, the resulting suspension is delivered to an associated lter 16, whereby the calcium. fluoride is separated therefrom, and the resulting iiltrate is delivered to an associated tank 17. The iiltrate delivered to the tank 17 contains substantially all of the nickel cations and all of the hypophosphite anions of the initial depleted plating solution, aside from certain small losses that are substantially entirely mechanical in character; however, this filtrate contains only about 10% to 25% of the phosphite anion concentration of the initial depleted plating solution, which phosphite anion content is in no way objectionable. Moreover, during the treatment, there is substantially no loss from the initial depleted plating so.

. lution of the lactic anions and propionic anions therein,

except for the small quantities thereof that are lost mechanically, as previously noted.

In the tank 17, the filtrate is reconstituted by the addition thereto of ynickel sulfate, sodium hypophosphite andy sulfuric acid, together with sodium hydroxide, to obtain the desired pH of about 4.7 in this illustrative bath. Also, small additions of lactic acid and propionic acid are made in order to reconstitute the platin-g bath with respect to these ingredients. At this time, the regenerated plating solution has substantially the composition of the initially formulated plating solution and is returned to the continuous plating system for further plating use.

` In the process, the calcium hydroxide is employed in conjunction with the calcium sulfate so as to provide substantially the required amount of calcium ions to precipitate the calcium phosphite, and to bring the solution to the optimum pH range, as noted above. Also, it is emphasized that the matter of removing the small excess of calcium ions from the filtrate from the filter 14 is optional, whereby a by-pass 18 is illustrated that extends directly from the filter 14 to the tank 17. A valve 19a is arranged in the by-pass 18, and a valve 19b is arranged in the direct conduit -to the tank 15, so that all, or any part, of the ltrate from the til-ter 14 may be diverted directly to the tank 17.

Referring now to Fig. 2, a conventional continuous chemical nickel plating system is diagrammatically illustrated as comprising a regeneration tank 21, a storage tank 22, a heater 23, a plating tank 24, and a cooler 25, arranged in tandem relation in the order named, together with a pump 27 and a lter 28 arranged in series relation between the storage tank 22 and the heater 23, and a.

.pump 29 arranged between the plating tank 24 and the .cooler 25.

In the `operation of the continuous plating system, the plating solution is lregenerated in the regeneration tank 21 (which may correspond in `fact to the tank 17 described in conjunction kwith Fig. l), and while the plating Asolution is cool and at a temperature of about 60 C. From the regeneration tank 21, the plating solution is delivered to the storage tank 22, from Which-it is pumped by the pump 27 through the filter 28 and thence through the heater 23 and delivered to the plating tank 24. In the heater 23, the-plating solution is heated tofan effective plating temperature in the general range 85 C. to 95 C. From the plating tank 24, the plating solution is-pumped by the pump 29 through the cooler 25 and returned to the regeneration tank 21. In the cooler 25, the plating solution is cooled so that the temperature thereof is returned substantially back to about 60 C., prior to the return of the plating solution to the regeneration tank 21. Of course, the work-pieces to be plated are immersed in the `plating solution in the plating tank 24 inthe usual manner; which work-pieces must necessarily have a catalyticsurface so that the plating reaction may proceed. In :this connection, it is mentioned that thecatalytic elements are: cobalt, nickel, palladium, rhodium and ruthenium. VHowever, alarge group of other elements can be rendered catalytic, either by displacement orrby galvanic Ainitiation in the -plating solution, so that the autocatalytic plating reaction may proceed. This group of elements includes aluminum, carbon, copper, iron, magnesium, titanium and uranium (as Well as silver and gold), and the various alloys thereof. Also, insulators can be satisfactorily nickel plated when the surfaces thereof are suitably activated Iwith one of the `catalytic elements named.

In-the continuous chemical nickel plating system aplating solution that has been regenerated in accordance With fthe present lprocess is equally effective as an initially composed plating solution in the production of bright, smooth coatings upon thework-pieces, that are intimately bonded thereto, and with a high plating rate. Specifically, in the present example, the plating rate is normally in the range of 0.9 to 1.0 mil/hour of nickel plating upon the Iwork-pieces.

In applying thepresent process of regeneration to the continuous .plating system, the whole body of plating solution may be regenerated after several passes thereof through the system, or a fractional part thereof may be by-passed continuously from the cooler 25 to the regeneration equipment, followed by the present treatment and then the return thereof to the regeneration tank 21.

The Vfollowing procedure is illustrative of repeated regenerations of an aqueous 'chemical nickel plating solution of the Vnickel cation-hypophosphite anion type. Specifically, an initial plating solution of this type was provided and after utilization thereof in the continuous plating system, it became depleted, at which time it had the following composition with respect to the important lons:

M./1. N++ 0.093 H2PO3- HPO3- 1.23 S04- 0.36

This depleted plating solution was regenerated in accordance with the previously described process; whereby it was first cooled to a temperature of 5 C. to elfect the crystallization of sodium sulfate therein. The solution was then ltered to remove the sodium sulfate; and thereafter to the filtrate there was added an aqueous slurry of 0.35 m./l. CaSO.1.1/2H2O and 0.65 m./1. Ca(OH)2, so as to effect the precipitation of calcium phosphite therein. The suspension was then filtered to remove the precipitated calcium phosphite therefrom;

6 Awhereby the .ltrate had vthe following composition -with respect -to ythe 'important ions: /l

This depleted plating solution was regenerated in-ac cordance -with Ithe previously described process; whereby it was rs`t cooledto'atemperature:of.5C. to effect 'the crystallization of sodium sulfate therein. The suspension was then'ltered to remove `the sodium sulfate; and therea'fter to vthe filtrate there was added an aqueous slurry "of 0.35 m./l. :of CaSO4.1/2H2O and 0.65 `m./l. CaO, so vas to eiect'the precipitation of calcium phosphite therein. The suspension'was then filtered to remove the .precipitated calcium :phosphite therefrom; and to the filtrate there was added 0.04 m./l. of NaF, so as vto effect the vprecipitation of calcium fluoride therein. The suspension was then iiltered'to remove lthe. precipitated y.calcium fluoride therefrom; whereby the filtrate had the following -composition with .respect to theimportantions:

Ni+ H2PO2- HP O3- s O4"- Ca++ Negligible employed; whereas in the second case, calcium oxide was employed; and it will be understood that the utilization of these two compounds is interchangeable, as each provides calcium cations and effects the desired increase in the pH of the solution. Further it is mentioned that inthe second regeneration set forth, sodium fluoride was employed in order to remove the residual calcium cations, as calcium fluoride, as previously explained.

In the foregoing examples ofthe present process, it is mentioned that about 75% to 95% of the phosphite anions are removed by the calcium phosphite precipitation; and also about 20% of the nickel cations and about 10% of the hypophosphite anions are removed substantially entirely mechanically, rather than by chemical precipitation, as there is no precipitation of any nickel salt. Specifically, the loss of nickel cations and hypophosphite anions is accounted for largely by mechanical trapping and the consequent removal along with the sodium sulfate, the calcium phosphite and the calcium fluoride. Also, it is pointed out that, when thel sodium sulfate is crystallized out, it carriesfrom 7 to` 10 molecules vof Water of crystallization therewith, andl when the calcium phosphite is precipitated, it carries 3 molecules of water of vcrystallization therewith; whereby the process actually -eiectsa concentration of the depleted solution notwithstanding the utilization of the aqueous slurry in the tank 13.

In the foregoing description of the present process, the regeneration of a plating solution has been described in conjunction with the removal of calcium phosphite utilizing calcium sulfate and calcium hydroxide, and this alkaline earth salt and alkaline earth hydroxide are preferred as a matter of simplicity and economy. However, other alkaline earth salts and alkaline earth hydroxides may be employed to precipitate the corresponding alkaline earth phosphite, when the plating solution is initially composed with a nickel salt, other than nickel sulfate. For example, the plating solution may be initially composed utilizing nickel chloride and sodium hypophosphite; Vwhereby in this case, the alkaline earth salt may be barium sulfate, barium chloride, etc., strontium sulfate, strontium chloride, etc., as well as calcium sulfate, calcium chloride, etc.; and likewise, the alkaline earth hydroxide may be barium hydroxide, strontium hydroxide or calcium hydroxide.

The procedure utilizing the other alkaline earth salts and alkaline earth hydroxides is identical `to that previously described; and it is preferable that the optimum range of pH 5.5 to 6.0 be employed, as it is this pH range that prevents the precipitation of nickel phosphite. Also, in this connection, it is mentioned that the various alkaline earth hydroxides are substantially equally elective to bring about the precipitation of the corresponding alkaline earth phosphites, without the precipitation of the corresponding alkaline earth hypophosphites.

It is reiterated that when the bath is initially composed with nickel sulfate, then the regeneration must take place with acalcium salt and calcium hydroxide, since it will be'immediately apparent that the utilization of barium or strontium salts and hydroxides would be primarily effective to bring about the precipitation of the corresponding barium or strontium sulfates and secondarily effective to bring about the precepitation of the corresponding barium or strontium phosphites. In other words, barium sulfate is far less soluble than barium phosphite, and strontium sulfate is far less soluble than strontium phosphite. However, this is not true of other barium salts (barium chloride) and of other strontium salts (strontium chloride). On the other hand, calcium is unique, as the sulfate thereof is relatively soluble, and so is the chloride; whereby the regeneration procedure using calcium salts and calcium hydroxide has universal application to these plating solutions.

Another consideration is of importance in the regeneration of these plating solutions, as a practical matter, iin that it is highly desirable to prevent the introduction of anions that are foreign to those already present therein. By way of illustration: calcium sulfate is employed when the plating solution is composed with nickel sulfate; alkaline earth chloride is employed when the plating solution is composed with nickel chloride; etc.

Furthermore, it will be understood that while the description has proceeded in terms of the utilization of alkaline earth sulfates and alkaline earth chlorides, the other soluble alkaline earth salts are equally effective. Moreover, while the description has proceeded in terms of the utilization of yalkaline earth hydroxides, the corresponding alkaline earth oxides and carbonates are equally effective.

In view of the foregoing, it is apparent that there has been provided an improved process of regenerating a depleted aqueous chemical nickel plating solution of the' nickel cation-hypophosphite anion type, wherein the desirable nickel cations and hypophosphite anions are maintained in the solution, while the undesirable phosphitev anions are removed therefrom; whereby the treated solution comprises an adequate and appropriate basis for ther reconstitution of a chemical nickel plating solution having substantially the initially formulated composition.

While there has been described what is at present considered to be the preferred embodiment of the invention, -it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall Within the true spirit `and scope of the invention.

What is claimed is:

1. The process of regenerating a depleted aqueous chemical nickel plating solution containing desirable nickel cations and hypophosphite anions and undesirable phosphite anions and having a pH below about 5.5, comprising adding to the depleted plating solution both an alkaline earth hydroxide and an alkaline earth salt of a mineral acid, said addition supplying suicient hydroxyl ions to the resulting solution to increase the pH thereof into the range 5.5 to 7.0, but not above, so as to precipitate therein alkaline earth phosphite, without precipitating therein any substantial amounts of alkaline earth hypophosphite or of any nickel compound, said addition supplying sufficient alkaline earth cations to the resulting solution to effect the removal therefrom of a substantial proportion of the phosphite anions therein, removing the precipitated alkaline earth phosphite from the resulting solution, and then adding to the resulting solution the required nickel cations and hypophosphite anions and hydrogen ions to produce a plating solution of desired composition.

2. The process of regenerating a depleted aqueous chemical nickel plating solution containing desirable nickel cations and hypophosphite anions and undesirable' phosphite anions and having a pH below about 5.5, comprising adding to the depleted plating solution both calcium hydroxide and a calcium salt of a mineral acid, said addition supplying sufficient hydroxyl ions to the resulting solution to increase the pH thereof into the range 5.5 to 6.0, but not above, so as to precipitate therein calcium phosphite, without precipitating therein any substantial amounts of calcium hypophosphite or of any nickel compound, said addition supplying sufficient cal' cium cations to the resulting solution to eiect the removal therefrom of a substantial proportion of the phosphite anions therein, removing the precipitated calcium phosphite from the resulting solution, and then adding to the resulting solution the required nickel cations andhypophosphite yanions and hydrogen ions to produce a plating solution of desired composition.

3. The process of regenerating a depleted aqueous chemical nickel plating solution containing desirable nickel cations and hypophosphite anions and undesirable phosphite anions and having a pH below about 5.5, comprising adding to the depleted plating solution both calcalcium hypophosphite or of `any nickel compound, saidv addition supplying suicient calcium cations to the resulting solution to effect the removal therefrom of a substantial proportion of the phosphite anions therein,

removing the precipitated calcium phosphite yfrom they resulting solution, and then adding to the resulting solution the required nickel cations and hypophosphite anionslv and hydrogen ions to produce a plating solution of desired composition.

4. The process set forth in claim 3, wherein the total amount of calcium cations added to the depleted solution; is not in excess of about of the stoichiometric amount thereof required to precipitate all of the phosphite therein.

5. The process of regenerating a depleted aqueous chemical nickel plating solution containing desirable` nickel cations and hypophosphite anions and undesirable phosphite anions and having a pH below about .5, comprising adding to the depleted plating solution both calcium hydroxide and calcium sulfate, said addition supplying sucient hydroxyl ions to the resulting solution to increase the pH thereof into the range of 5.5 to 6.0, but not above, so as to precipitate therein calcium phosphite, without precipitating therein any substantial amounts of calcium hypophosphite or of any nickel compound, said addition supplying suicient calcium cations to the resulting solution to effect the removal therefrom of a substantial proportion of the phosphite anions therein, removling the precipitated calcium phosphite from the resulting solution, adding to the resulting solution sodium uoride so as to precipitate therein calcium uoride, removing the precipitated calcium uoride from the resulting solution, and then adding to the resulting solution the required nickel cations and hypophosphite anions and hydrogen ions to produce a plating solution of desired composition.

6. 'I'he process of regenerating a depleted aqueous chemical nickel plating solution containing sulfate anions, and desirable nickel cations and hypophosphite anions and undesirable sodium cations and phosphite anions and having a pH below about 5.5, comprising stripping from the depleted plating solution a substantial amount of the sodium cations and -the sulfate anions therein as sodium sulfate, adding to the resulting solution both calcium hydroxide and calcium sulfate, said addition supplying sutlcient hydroxyl ions to the resulting solution to increase the pH thereof into the range 5.5 to 7.0, but not above, so as to precipitate therein substantially only calcium phosphite, said addition supplying suiiicient calcium cations to the resulting solution to effect the removal therefrom of a substantial proportion of the phosphite anions therein, removing the precipitated calcium phosphite from the resulting solution, and then adding to the resulting solution the required nickel sulfate and sodium hypophosphite and hydrogen ions to produce a plating solution of desired composition.

7. The process set forth in claim l, wherein said alkaline earth hydroxide is barium hydroxide.

8. The process set forth in claim l, wherein said alkaline earth salt is calcium salt.

9. The process set forth in claim 1, wherein said alkaline earth salt is barium salt.

10. The process set forth in claim 1, wiherein said alkaline earth salt is calcium sulfate.

11. The process set forth in claim 1, wherein said alkaline earth hydroxide is strontium hydroxide.

12. The process set forth in claim 1, wherein said a1kaline earth salt is strontium salt.

References Cited in the le of this patent UNITED STATES PATENTS Spaulding Dec. 13, 1955 OTHER REFERENCES 

1. THE PROCESS OF REGENERATING A DEPLETED AQUEOUS CHEMICAL NICKEL PLATING SOLUTION CONTAINING DESIRABLE NICKEL CATIONS AND HYPOPHOSPHITE ANIONS AND UNDESIRABLE PHOSPHITE ANIONS AND HAVING A PH BELOW ABOUT 5.5, COMPRISING ADDING TO THE DEPLETED PLATING SOLUTION BOTH AN ALKALINE EARTH HYDROXIDE AND AN ALKALINE EARTH SALT OF A MINERAL ACID, SAID ADDITION SUPPLYING SUFFICIENT HYDROXYL IONS TO THE RESULTING SOLUTION TO INCREASE THE PH THEREOF INTO THE RANGE 5.5, TO 7.0, BUT NOT ABOVE, SO AS TO PRECIPITATE THEREIN ALKALINE EARTH PHOSPHITE, WITHOUT PRECIPITATING THEREIN ANY SUBSTANTIAL AMOUNTS OF ALKALINE EARTH HYPOPHOSPHITE OR OF ANY NICKEL COMPOUND, SAID ADDITION SUPPLYING SUFFICIENT ALKALINE EARTH CATIONS TO THE RESULTING SOLUTION TO EFFECT THE REMOVAL THEREFROM OF A SUBSTANTIAL PROPORTION OF THE PHOSPHITE ANIONS THEREIN, REMOVING THE PRECIPITATED ALKALINE EARTH PHOSPHITE FROM THE RESULTING SOLUTION, AND THEN ADDING TO THE RESULTING SOLUTION THE REQUIRED NICKEL CATIONS AND HYPOPHOSPHITE ANIONS AND HYDROGEN IONS TO PRODUCE A PLATING SOLUTION OF DESIRED COMPOSITION. 